Locating roof crossbeams for solar panel install?

I used four 6" long Z brackets with three SS screws per bracket (making a total of 12 screws per panel) and lots of 3M 4200 adhesive/sealant under the Z brackets.

And didn't pay much attention to finding the rafters, was rather more concerned that the Z brackets were properly placed along the solar panels (so the panels wouldn't bend and flex much), and with panel placement to reduce shading by the A/C and vents.

So far, several thousand km's later, no problems.